The feline leukemia virus and the feline immunodeficiency virus are both pathogens that affect the feline population worldwide. Both are retroviruses and infection with either means a lifelong disease sentence. There is no cure for either disease.
FeLV affects about 3% of domestic cats and rates can be as high as 11% in feral and stray feline populations. The virus is contagious and transmitted directly though saliva and blood. Kittens can also be affected if their mother is infected before birth. The virus affects each individual cat in different ways – some cats resist infection, some become visibly sick and some others harbor the virus but show no symptoms. Common outcomes of infection are due to immune system suppression and can include degenerative diseases and cancer. Most cats eventually succumb to secondary disease and die.
FIV also affects the immune systems of cats. It is estimated that anywhere from 2-4% of cats in the US are infected with FIV. Transmission is similar to FeLV, with the main modes being blood and mother-to-kitten. Unlike FeLV, the outcome of infection is always the same – the cat retains the virus and becomes sick. Symptoms are also related to immune suppression, with opportunistic infections often being fatal in infected cats.
Many cats infected with FeLV or FIV can live long, relatively healthy lives. However, these cats are generally confined indoors with no contact to other cats. This decreases the risk of opportunistic infections and prevents the cat from infecting other felines.
Both of these diseases are a very large concern in populations of feral and stray cats. Males are often at a higher risk due to their roaming and fighting tendencies. In cases where feral and stray cats are found to be infected with FeLV or FIV, humane euthanasia is generally accepted as the best solution. Unfortunately many of these cats would never receive the proper medical attention and nutritional needs necessary for them to survive. In this way, the infected cats are not left to die due to secondary infections and the spread of these fatal diseases is slowed.
Statistics and information taken from http://www.winnfelinehealth.org/health/felv-fiv.html.